The xylenes, namely orthoxylene, metaxylene and paraxylene, are important chemicals and find wide and varied application in industry. Orthoxylene is a reactant for the production of phthalic anhydride. Metaxylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Paraxylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers.
As a result of the important applications to which the individual xylene isomers are subjected, it is often very important to be able to produce high concentrations of a particular xylene. This can be accomplished by converting a nonequilibrium mixture of the xylene isomers, which mixture is low in the desired xylene isomer, to a mixture which approaches equilibrium concentrations. Various catalysts and processes have been devised to accomplish the isomerization process. For example, it is well known in the art that catalysts such as aluminum chloride, boron fluoride, liquid hydrofluoric acid, and mixtures of hydrofluoric acid and boron fluoride can be used to isomerize xylene mixtures.
Another well-known xylene isomerization catalyst is amorphous silica alumina, having the trade name "Durabead".
A typical processing scheme for producing paraxylene comprises:
(a) isomerizing a C.sub.8 alkylaromatic mixture to near equilibrium in an isomerization reaction zone;
(b) separating out paraxylene as by low temperature crystallization, to obtain a paraxylene-rich stream and a stream rich in other xylenes; and
(c) recycling the stream rich in other xylenes to the isomerization reaction zone.
The present invention is particularly concerned with the isomerization reaction step which may be used in an overall process directed to paraxylene production.
Numerous catalysts have been proposed for use in xylene isomerization processes such as mentioned above. More recently, a number of patents have disclosed the use of catalysts containing molecular sieves or zeolites for isomerization of C.sub.8 alkylaromatics. For example, U.S. Pat. No. 3,790,471 discloses the use of a zeolite identified as ZSM-5 for hydroconversions such as isomerization of polyalkyl substituted aromatics, for example, orthoxylene.
ZSM-5 is a crystalline aluminosilicate zeolite having intermediate size pores. Various zeolites having intermediate size pores are described in commonly assigned U.S. patent application Ser. No. 305,679 of S. J. Miller. Ser. No. 305,679 is not directed to isomerization but rather is directed to oligomerization of olefins using a catalyst comprising ZSM-5 impregnated with zinc or cadmium.
Isomerization reactions using crystalline aluminosilicate zeolite catalysts may be carried out in the presence of added hydrogen. See for example, U.S. Pat. No. 4,331,822, Onodera et al., which discloses vapor phase isomerization in the presence of added hydrogen using a crystalline aluminosilicate zeolite such as ZSM-5 and wherein the zeolite contains at least two metals which are (a) platinum and (b) at least one metal selected from the group consisting of titanium, chromium, zinc, gallium, germanium, strontium, yttrium, zirconium, molybdenum, palladium, tin, barium, cesium, cerium, tungsten, osmium, lead, cadmium, mercury, indium, lanthanum, beryllium, lithium and rubidium.
The process described in U.S. Pat. No. 4,331,822 is carried out at pressures in the range of 100 to 200 psig in the presence of hydrogen. U.S. Pat. No. 3,281,482 discloses isomerization using a crystalline aluminosilicate of the zeolite type at pressures preferably of 500 to 1000 psig.
U.S. Pat. No. 3,856,873 discloses an isomerization reaction operating at pressures of about 0 to 1000 psig, and temperatures of about 500.degree. F. to 1000.degree. F., maintaining vapor phase reaction conditions. The catalyst used in the '873 process is a ZSM-5 zeolite. The zeolite can be used alone or with an added metal such as nickel. Typically, the zeolite used in the '873 process also is combined with alumina, the alumina serving as a binder material in which the zeolite is embedded to form an attrition-resistant catalyst pellet.
U.S. Pat. No. 3,856,873 points out at column 6, line 40, that, "Since the process is conducted in the absence of added hydrogen, there is no need for metals of the transition group such as nickel, platinum, palladium, etc. These metals may be present, but as now understood, the process appears to be unaffected by such cations."
Other patents directed to xylene isomerization using zeolitic catalysts with added hydrogen include U.S. Pat. No. 4,163,028, which discloses use of ZSM-5 having a silica to alumina ratio of at least 500 and wherein the isomerization temperature is greater than 800.degree. F.; U.S. Pat. No. 3,856,872, which discloses isomerization using ZSM-5, ZSM-11, or ZSM-12; U.S. Pat. No. 4,218,573, which discloses use of ZSM-5 containing alkali metal cations such as sodium; and U.S. Pat. No. 4,101,596, which discloses isomerization without added hydrogen, using a ZSM-5 catalyst at a pressure less than 100 psig, a temperature between 500.degree. F. and 800.degree. F., and using an alkylaromatic feed which is essentially free of peroxide. U.S. Pat. No. 4,159,282 to Olson discloses a xylene isomerization process that preferably is carried out in the presence of hydrogen. Olson uses a catalyst having a crystalline aluminosilicate zeolite with a crystal size of at least 1 micron, and ZSM-5 is a preferred aluminosilicate for use in his process. To control xylene sorption characteristics of his catalyst, preferably P, B, Mg, or Sb oxides are included in the catalyst. Example 7 uses 6.0 weight percent Mg O, which is about 3.6 weight percent expressed as Mg.